Is it ethical to have results in your PhD thesis that are not strictly yours?

Question

In some disciplines (such as theoretical computer science), it is typical to have papers where you have co-authors. Especially when the set of co-authors is large, and work is being done at different universities, it is possible that person X works on one part, and person Y on one part. Thus, perhaps you wrote and came up with one result, and another result was the result of the work of someone else. Finally, you publish that paper together.

Then, in your PhD thesis, you typically have a list of publications. From what I've seen, the author usually declares what is his/her contribution was for each publication, but this is quite vague. For instance, it might say e.g., "Publication 1 is joint work with X and Y, and partly written by the author".

So consider a particular result that appears in a paper where you are a co-author as a PhD student. My question(s) are:

1. Is it ethical to include that result in your PhD thesis, supposing that you did not contribute to the result? Perhaps you only proof-read the theorem.
2. Does it make a difference if you couldn't have come up with the result, or don't deeply understand it? Maybe professor Bigshot Superstar came up with it, wrote it, and it is using advanced machinery with several technical details.

It is nice to just say "this is joint work with Bigshot Superstar" to make yourself look good. I see it being done, but it makes me feel uneasy. On the other hand, while it seems honest, it can potentially hurt you to say "the author did not contribute to Theorem X".

Answer 1

It may be good to report results that is connected to your reports in your thesis even if you did not directly contribute to it as long as you attribute main contributor of the result from an ethical stand-off. But this would not be applicable if the concerned result is to play a major role in your dissertation. I which case, IMO, you try to do something more or different to make up for the major result.

Apart from my views, the your thesis advisor ought to be the best person to clarify this particular doubt.

Answer 2

As you noted yourself, it is fairly common to have coauthors on papers written during your PhD. But this should not prevent these results from counting towards your degree.

How this was handled when I did my PhD (math in Denmark) was that I included what was essentially a copy of a paper I wrote with a coauthor (though with notation changed to match, and actually only about half the paper as that was the relevant part for the rest of the dissertation), with a note at the beginning of that section stating that this was what it was.
I also had to get a declaration from my coauthor that he was ok with this, which also stated how large my contribution to the work was. This was then sent along with the dissertation to the evaluation committee.